1. Technical Field
This invention relates to heterojunction bipolar transistors, and more particularly to techniques for producing high reliability GaAs/AlGaAs heterojunction bipolar transistors.
2. Discussion
Gallium arsenide/aluminum gallium arsenide heterojunction bipolar transistors (HBTs) are utilized in many important microwave applications such as log amps, analog to digital converters, and sample and hold circuits. Most commonly, such HBTs are produced by Molecular Beam Epitaxy (MBE) using beryllium as the p-type dopant. However, due to the intrinsic difficulties associated with beryllium incorporation and diffusion during MBE growth, HBTs produced by MBE with beryllium doping have generally had poor reliability.
Part of the problem is that it is difficult to grow HBT profiles that are well-defined. Moreover, even when they are well-defined as grown, after processing the beryllium p-type dopant moves from the base to the AlGaAs emitter when under bias at high temperature. This limits the usefulness of these devices for applications that require relatively high reliability components. In particular, the base-emitter p-n junction must be well-defined and coincident with the base-emitter heterojunction to avoid increased V.sub.be, and reduced beta or gain.
The primary cause of rapid beta (B) degradation in conventional HBT structures is interstitial Be.sup.+ diffusion from the base to the emitter, similar to interstitial Zn.sup.+ diffusion in tunnel diodes. When the beryllium in located on interstitial lattice positions, it becomes a positively charged donor, and diffuses easily. What is desired is for the beryllium to enter substitutional lattice sites taking the place of the gallium atom on a gallium lattice site, in which case the beryllium will become an acceptor, will have strong bonding to the GaAs lattice, and will not diffuse easily.
Increasing the As/Ga flux ratio has been shown to help maintain sharp beryllium profiles, both during growth and subsequent annealing by increasing the concentration of gallium arsenide vacancies in the crystal. However, acceptable reliability has still not been achieved.
Thus, it would be desirable to provide a heterojunction bipolar transistor which is reliable and which maintains a high B over time. Further, it would be desirable to provide a method for the incorporation of beryllium during MBE growth which results in well-defined HBT profiles with minimal diffusion of beryllium p-type dopant from the base to the emitter when under bias at high temperatures.